There are known plasma-arc torches comprising each a body, a nozzle, an electrode holder carrying a central electrode, and means for supplying electric current, water and gas.
Tungsten is the most suitable material for the electrode of such a plasma-arc torch which utilizes straight- or reverse-polarity operating mode and has its electrode surrounded by inert or neutral gases.
In these plasma arc torches, maximum permissible currents depend on the rate of electrode cooling. In the case of straight-polarity operating mode (with the tungsten electrode used as a cathode) the cathode is heated relatively weakly. Therefore, at working currents of several hundreds of amperes, conventional grips disposed in a water cooled electrode holder are adequate.
However, grips are mainly used in low-powered plasma-arc torches utilizing monatomic gases as the working medium. This is due to the fact that the linear contact between the grip and the electrode, and between the grip and the water-cooled electrode holder, provides a relatively high electrical and heat resistance with the result that the grip cannot resist large electrical and thermal loads.
A high thermal resistance leads to intensive heating of both the grip and the electrode; this occurs at currents exceeding 1000 A and results in excessive wear of the electrode.
The heating of the grip also deteriorates its mechanical strength and disturbs contact between the electrode, the grip and the water-cooled electrode holder. This, in turn, leads to an increase in the electrical and thermal resistance of the grip and to further heating of all its parts, and, finally, the grip fails.
In addition, insufficient cooling of the grip does not allow for a decrease in the ratio of the free portion of the electrode to its diameter, which can bring the working surface of the electrode close to the water-cooled electrode holder. The ratio l/d is a critical design parameter of the plasma-arc torch, responsible for the intensity of cooling of the electrode working surface, where l is the electrode length, and d is the electrode diameter. It will be appreciated that the smaller the value of this ratio, the greater will be the cooling of the working surface of the electrode and the smaller the wear of the electrode. In the known plasma-arc torches with grips, minimum ratio l/d is 1/2. At smaller values of this ratio, the grip melts off.
The use of the nitrogen-hydrogen mixture as the working medium and application of higher currents requires that the electrode be cooled intensively. For this, use is made of tungsten electrodes soldered in a water-cooled electrode holder made of copper. An electrode assembly with soldered-in electrodes allows the electrode to be cooled much more intensively with the result that lesser wear of the electrode takes place owing to a relatively good contact between the surfaces of the electrode and the electrode holder. In addition, such a construction permits the working surface of the electrode to be close to the cooling area so that the ratio l/d is decreased due to intensive cooling of the electrode holder.
However, the construction with soldered-in electrodes is disadvantageous since solders are maintained here at a relatively low melting temperature of 300 to 1200.degree. C. and include elements featuring a higher vapour pressure. This does not allow the ratio l/d to drop off down to the required value as otherwise the elements of a solder having a higher vapour pressure evaporate selectively and the joint between the cathode and the electrode holder is not, therefore, reliable and fully sealed. Apart from this, in the area where the electrode is soldered to the electrode holder (whatever solder is used) there results a brittle intermetallic compound with the result that the joint features a considerable brittleness and low mechanical strength, and cracks often appear in the soldering area due to thermal shocks.
There are also known electrode assemblies having pressed-on or screwed-in electrodes, in which the above-described disadvantages of their predecessors have been eliminated to some extent.
In the case of pressed-on electrodes, for example, the difference between the linear expansion coefficients of the materials of the electrode and the electrode holder results in the deterioration of the mechanical strength of the pressed-on connection and in poor contact between the electrode and the electrode holder each time they are heated. Also the tightness of the water-coolant passages of such electrode assemblies become worse, while the surfaces of the pressed-on connection are constantly subject to natural oxidation which leads to poor electrical and thermal conduction at the joint between the electrode and the electrode holder. In the case of screwed-in electrodes, similar disadvantages are obviously encountered.
Thus, the known electrode assemblies cannot utilize currents that considerably exceed permissible limits. In the case of straight-polarity operating mode, such currents result in low operational effectiveness, while in the case of reverse-polarity operating mode, inadequate heat transfer conditions are observed which do not allow for the use of tungsten electrodes since they are heated to a temperature which is 8 to 10 times that noted in straight-polarity applications.
There is an electrode assembly for plasma-arc torches (cf. British Pat. No. 1,402,199), comprising an electrode holder made of copper or copper alloys and having a butt portion which rigidly secures a tungsten electrode. This attachment is accomplished as follows. The electrode is immersed in the molten metal, for example, copper or a copper alloy, from which the electrode holder is to be made. Prior to immersing the electrode in the molten pool, they are coupled to a d.c. circuit to strike an arc therebetween. An extended contact between tungsten and copper is provided so that the electrode is given a shape which ensures good heat exchange.
However, the mechanical strength of the joint between the electrode and the electrode holder is insufficient in the presence of high electrical and thermal loads. Since tungsten is insoluble in the molten copper (cf. M. Hansen, Constitution of Binary Alloys, Second Edition prepared with the cooperation of Kurt Anderko, McGraw-Hill Book Company, Inc., New York-Toronto-London, 1958, p. 649) there is no bonding at the joint between the electrode and the electrode holder. In operation, the electrode assembly is subject to considerable thermal shocks causing the extension of the metals from which the electrode and the electrode holder are made. Due to the difference between the linear extension coefficients of copper and tungsten, their joint is destroyed so that clearances appear, the metals oxidize and electrical and thermal conduction deteriorate, which leads to further destruction of the joint. As a result, the actual life of the electrode assembly is reduced. When plasma-arc torches are used for metal cutting and when a limited number of switching-ons (say, 20) is employed, the above-described disadvantages do not considerably affect the actual life of the electrode assembly. In the case of metal-melting applications, however, plasma-arc torches may operate 1000 hours and more, which requires that the metals of the electrode and the electrode holder be jointed reliably.